JP 2003-164039A discloses an electrical junction box that is connected to a battery and a load, such as a head lamp or a wiper. In this electrical junction box, a plurality of plate-shaped conductors are placed on a placement surface of a heat dissipation member with an insulation member interposed therebetween, and plate surfaces on one side of the conductors face the placement surface. One end and another end of a semiconductor switch are respectively connected to two of the plurality of conductors. The one end of the semiconductor switch is connected to the battery via one of the two conductors, and the other end of the semiconductor switch is connected to the load via the other of the two conductors.
A circuit board is arranged on plate surfaces on the other side of the plurality of conductors, and a plate surface of the circuit board faces the plate surfaces on the other side of the conductors. A control element is arranged on the other plate surface of the circuit board, and the control element outputs control signals for turning the semiconductor switch ON or OFF. If the control element outputs a control signal for turning the semiconductor switch ON, the semiconductor switch is switched ON and power is supplied from the battery to the load with current flowing via the semiconductor switch. If the control element outputs a control signal for turning the semiconductor switch OFF, the semiconductor switch is switched OFF and power supply from the battery to the load is stopped with the flow of current via the semiconductor switch being interrupted.
When current is flowing via the semiconductor switch, the semiconductor switch generates heat. The heat generated by the semiconductor switch is conducted by the conductors and the heat dissipation member in this order, and dissipates from the heat dissipation member.
In the electrical junction box described in JP 2003-164039A, the heat dissipation member, the conductors, the circuit board, and the control element are arranged in this order. Therefore, heat generated by the semiconductor switch is conducted via the conductors and the circuit board to the control element. Accordingly, if the temperature of the semiconductor switch increases, the temperature of the control element increases at a speed that is substantially the same as the speed at which the temperature of the semiconductor switch increases.
The control element has low heat resistance, and therefore the upper limit value of the temperature of the semiconductor switch up to which its normal operation is ensured is usually higher than the upper limit value of the temperature of the control element up to which its normal operation is ensured. In this case, operation of the semiconductor switch is controlled such that the temperature of the semiconductor switch does not become higher than or equal to the upper limit value of the temperature of the control element. For example, if the temperature of the semiconductor switch becomes close to the upper limit value of the temperature of the control element, the control element outputs a control signal to switch the semiconductor switch OFF and stop power supply via the semiconductor switch.
As described above, the electrical junction box described in JP 2003-164039A has a problem in that the upper limit value of an allowable temperature of the semiconductor switch is lower than the actual upper limit value of the temperature of the semiconductor switch.
Circuit devices in which such a problem occurs are not limited to the electrical junction box described in JP 2003-164039A. Similar problems occur in circuit devices in which a heat dissipation member, a conductor, a circuit board, and a control element are arranged in this order and a circuit component that generates heat is connected to the conductor.
In such a circuit device, the control element outputs control signals for controlling operation of the circuit component, and the upper limit value of the temperature of the circuit component up to which its normal operation is ensured is higher than the upper limit value of the temperature of the control element up to which its normal operation is ensured. Therefore, the upper limit value of an allowable temperature of the circuit component is lower than the actual upper limit value of the temperature of the circuit component.